Plesiochronous

Pronounced plee-see-AH-krun-us (from the Greek plesos, meaning close; and chronos, meaning time) a term describing a condition where two or more signals are nearly synchronized. For example, a communication system of digital devices where the clocks between a transmitting and receiving device aren’t locked directly to one another (as in a master and slave relationship), but are both highly accurate and are running at the “same” rate (same, meaning, in literal terms, “nearly” the same). There are standards governing the rate of deviation allowed before a system can be considered plesiochronous. Short of that it would be asynchronous. One real world example of a plesiochronous system might be two word clock generators that both are deriving their clock rate from a common black burst generator. We usually would think of this as a synchronous system (and it generally will work), but depending upon how accurate the two generators really are it could fall into the plesiochronous category. Similarly, two word clock generators running on their own would tend to be an asynchronous system, but could, if accurate enough, fall into the plesiochronous range.